Diphenylsulfone cross-linking compounds represented by the following formula (IV) are known as a color-developing agent or an image storage stabilizer (e.g., see Patent Document 1).
[wherein X and Y may each be different from one another, and each either represents a C1-C12 hydrocarbon group which may be linear or branched, saturated or unsaturated and which may have an ether bond, or represents the following formula
(wherein R′ represents a methylene group or an ethylene group and T represents a hydrogen atom or C1-C4 alkyl group); R1-R6 each independently represents a halogen atom, C1-C6 alkyl group or C2-C4 alkenyl group; m, n, p, q, r and t each represents an integer of 0 to 4 and when representing an integer of 2 or more, R1-R6 may each be different; and “a” represents an integer of 0 to 10]
These compositions, however, had insufficient heat resistance although exerting a remarkably superior storage property compared to the conventional compositions when used for a recording material. Also, as to a mixture of diphenylsulfone cross-linking compounds produced by, for example, reacting 4,4′-dihydroxydiphenylsulfone with bis(2-chloroethyl)ether, it was not easy to remove the raw material 4,4′-dihydroxydiphenylsulfone that remained unreacted.
Since 4,4′-dihydroxydiphenylsulfone referred to above is currently designated as a Type II Monitoring Chemical Substance set forth in the “Law Concerning the Examination and Regulation of Manufacture, etc. of Chemical Substances”, it is desired to remove this compound as much as possible.    [Patent Document 1] Japanese Laid-Open Patent Application No. 10-29969